The invention concerns a method of resynchronizing a radio telephone system clock in relation to an associated transmitter/receiver station system clock in idle mode, a resynchronization device for resynchronizing a radio telephone system clock, and a radio telephone having such a resynchronization device.
According to the European digital cordless telephone system (DECT), a portable telephone in locked idle mode is to resynchronize its timing in relation to its fixed part with a predetermined frequency. A DECT signal is divided into time slots and frames, where a multiframe corresponds to 16 frames, and each frame consists of 24 slots (TDMA), each having 480 bits. As the fixed part transmits data at a bit rate of 1,152 Mbits/sec., 184320 data bits will be transmitted during a multiframe. Even the best system clock will have an inaccuracy corresponding to at least 6-7 bits during such a period. Of course, it is possible to resynchronize the timing of the portable telephone more frequently, but this requires that the telephone actively tries to resynchronize the timing more frequently than prescribed by the standard, which means that the telephone is active for a relatively longer time resulting in decreased standby time.
This inaccuracy increases to at least about 12 bits, when the system with the clock is in sleep mode without any form of clock correction or feedback. This inaccuracy in the estimation of the time slot start causes some serious problems. To be certain that the synchronization algorithm has started before the first bit in the preamble sequence, the synchronization algorithm must be started at bit position minus 12 (with 12 bit inaccuracy). This involves the risk that, at worst, the synchronization algorithm starts already at bit position minus 24. In that case, the synchronization algorithm will search for the preamble in pure noise (no carrier on the antenna), involving a non-negligible risk of the noise causing detection of a false preamble because of the frequency modulation and the IF filter bandwidth. Finally, the portable telephone will have difficulty in transmitting at the correct frequency to the fixed part, if the frame synchronization is not correct.